Moving target radar system



1953 F. CUNNINGHAM, JR 2,659,079

MOVING TARGET RADAR SYSTEM Filed Dec. 10, 1945 REsPoNsE I2 r lO V w r e I (RADIAL VELOCITY) RESPONSE c ll/ls FIG.3

S w(RADIALVELOCITY) i DELAY P AMPLIFIER 5: I AMPLIFIER L l -2I 23 24 2 26 I f I AMPLIFIER Q2 F cANcELLER I l l I I II DELAY P AMPLIFIER AMPLIFIER CANCELLER fi- \.2a 30 i TO INDICATOK I 21 AMPLIFIER T0 RECEIVER 42 43 44 45 402 STgRAGE sTokAsE a cANcELLATIoN AMPL'F'ER I GANGELLATICTE' AMPL'F'ER TO INDICATOR.

INVENTOR FREDERIC CUNNINGHAM .IIR.

WW 9 M.

ATTORNEY Patented Nov. 10, 1953 MOVING-TARGET RADAR SYSTEM.

Frederic Cunningham, Jr., Belmar, N; J., assignor to the United States of America as represented by'the Secretary of War Application December 10, 1945, Serial No. 634,078

7 Claims. (Cl. 3437.7)

The present invention relates to a radio object locating system which is adapted to distinguish fixed objects from moving objects, and it relates more particularly toa moving object selector.

which forms a component of such a system.

A radio object locating system whichis adapted to distinguish fixed objects from moving objects, and which presents on an indicator, signals representing moving objects only, is referred to as a movingtarget indication (MTI) system. In an MTI system, exploratory pulses of radiant energy are-transmitted in the usual manner, and in addition reference oscillations having a fixed phase with respect to each transmitted exploratory pulse are provided at the receiver. Object returned echo pulses are combined with the reference oscillations and. the two combine to a degree depending upon their relative phase. Echo pulses from stationary objects have a constant phase with respect to thereference oscillations, whereas echo pulses from moving objects have a continuously varying phase. The combined echo pulses and reference oscillations are detected to provide video pulses, and video pulses representin moving objects have a cyclical variation in amplitude whereas video pulses representing stationary objects have a substantially non-varying amplitude.

The rate of phase change and hence the rate of video pulse. amplitude variation is directly proportionaltothe radial velocity of the reflecting object. Thus the frequency of the modulation envelope of aplurality of video pulses is directly proportional to the radial velocity of the detected object.

An MTI system makes use of thisdifierence in pulse to pulse amplitude to provide an indication of moving objects only. This is accomplished by means of a moving object selector. The basic principle of any moving object selector is to comparesuccessive video pulses and to provide pulse signals whose. amplitude is substantially proportional to the diiference in amplitude between successive video pulses. Thus, as pulses representing stationary objects are non-varying, no output signals will be obtained therefore from the moving object selector, Whereas a residual output pulse signal will be obtained for signals representing moving objects.

There are two major ways of comparingsuccessive pulses. One is to use a delay type transmission line (or delay line as it is usually called) which is adapted to delay signals of electrical energy for a predetermined period of time and wherein a portion of each video pulse signal is delayed for a period of time substantially equal to the interval between successive pulses. The delayed pulse signal is then compared with a second and undelayed portion of each video pulse signal in order to obtain an output pulse signal whose amplitude is substantially proportional to the difference in amplitude-between successive video pulse signals. Comparison is achieved by phase inverting either the delayed pulseor the The other major method of a pulse comparison is to use a storage tube. Basically, such a tube consists of a glass envelope containing a storage plate element, a load impedance connected thereto, and means for generating an electron beam which bombards said storage plate element. Video signals to be stored are used to modulate theelectron beam and a signal is developed across the output load impedance, said signal being sub stantially proportional to the-difference in amplitude between successive video pulses. A moving object selector'employing such a storage tube is more fully described in a copending application, of Alfred G. Emslie, entitled fMoving Object Detection System, Serial No. 594,266, filed May 17, 1945, and issued June 20, 1950; as Patent No. 2,512,144.

One difficulty in using storage tubes is that th storage plate element often charges quite slowly when the electron beam scans rapidly and thus the charge does not readily follow small changes in signalamplitude, thus-resulting in poor signal discrimination. Poor discrimination also results if the directional beam ofexploratory radiant energy scans too rapidly. Under such circumstances the echo'pulses from stationary objects vary in amplitude and have the appearance of moving object echoes. In both delay line and storage tube type moving object selectors, difiiculty is encountered when signals are received from two different reflecting sources where one reflecting source is moving at but a slightly different rate than the other. In such an instance the discrimination is poor'as it is difiicult to distinguish one group of signals from the other.

It is an object of the present invention to provide a moving object selector which makes an MTI system more eifective in distinguishing between signals from aircraft and. those from other moving objects.

It is another object of the present invention to provide a storage tube type of moving object selector which reduces the poor discrimination due to rapid scanning of the beam of radiant energy.

It is still another object of the present invention to provide a moving object selector which provides an improved discrimination between signals from objects having slightly different radial velocities.

The invention, however, will be more fully understood from the following detailed description when taken into consideration with the accompanying drawing wherein:

Fig. 1 is a schematic diagram of successive echo pulses and which serves to give a better understanding of the principles of the present invention;

Fig. 2 is a series of graphs which enable a better understanding of the present invention;

Fig. 3 is a block diagram of one embodiment of the present invention employing a delay line type moving object selector; and

Fig. 4 is a block diagram of one embodiment of the present invention employing a storage tube type moving object selector.

An understanding of the basic principles of the present invention may be obtained by reference to Fig. 1. In the figure, 2, 3, 4 and 5 represent successive pulses whose modulation envelope is E=f(t). Assume that pulse 3 occurs at a time equal to and that pulse 4 occurs at a time n and further assume that pulse 3 is to be compared with pulse 4 in a moving object selector and that a signal proportional to the amplitude diiference between pulse 4 and pulse 3 will be obtained. Then the amplitude of pulse 3 may be represented as Eo=f(tn) But as the interval between successive pulses may be represented as At, E1 may be written as E1=f(to+At) Attention is directed to the fact that if a delay line type moving object selector is used At is substantially equal to the amount of delay introduced by the delay line.

The difference in amplitude AE between pulses 3 and 4 is then and E qg .f ro+m f m ish ia where f(t) is equal to the first derivative of f(t) Thus, if At is sufficiently small, the modulation envelope of a plurality of pulses forming the output of a moving object selector is substantially proportional to the first derivative of the envelope of a plurality of pulses applied to said selector.

In view of the foregoing, it is evident that if successive pulses are compared, and a residual pulse'is obtained proportional to the diiTerence in amplitude between successive pulses, and that if the interval between successive pulses is relatively small compared to a complete cycle of the envelope at :0

whether it be either of the delay line type or the storage tube type is substantially proportional to the first derivative of the original pulse modulation envelope. If the output of the moving object selector is then applied to a second moving object selector and the process repeated, the signals obtained as an output from said second selector will have a modulation envelope proportional to the second derivative of the modulation envelope of the pulses applied to the first moving object selector.

In a moving target indication system, the modulation envelope of the video pulses obtained by combining the echo pulses with reference oscillations is substantially sinusoidal. Thus, the modulation envelope f(t) may be written as where w is proportional to the frequency of the modulation envelope of a plurality of video pulses. Then =E w COS wt and as the modulation envelope of the pulse si nals obtained at the output of the first moving object selector is proportional to derivative of the modulation envelope of the input pulses applied to the first moving object selector.

This second derivative with respect to t is This represents the modulation envelope of a plurality of pulse signals where said modulation envelope has a peak amplitude which varies with the square of w. It is thu evident that the output from the second moving object selector gives a higher order of discrimination with respect to to than does the output from the first moving object selector.

This improved discrimination is shown graphically in Figs. 2a, 2b, and 20 wherein the ordinate in each instance represents the peak amplitude (i. e. response) of a modulation envelope of a plurality of pulses, and the abscissa represents the radial velocity. It will be recalled that the radial velocity is proportional to or, where w is equal to 21r times the frequency of the modulation envelope of the video pulses applied to the first moving object selector. Fig. 2a represents the response characteristic at the input to the first moving object selector, and merely means that although the frequency of the modulation envelope of the video pulses varies with w, the peak amplitude of the modulation envelope remains constant for all values of w and hence constant for all radial velocities. For example, at m the peak amplitude of the modulation envelope is equal to the peak amplitude of the modulation The peak amplitude only, and

ammo-ac:

notzthetslmisoidelwariationsmr thezenvelope are shownizrthefigure.

Fig.1 21): represents the response characteristic atithe :outputrofxafirstemoving object selector. Thisifiguretindicates tthat'ithe.peak'i'amplitude of the modulationienvelope '0f;a';plura1lty:nf :pulses which form :thetoutputifrom.said moving. object selector varies linearly-with.,.w. :By .way of example, if :the rfrequency of "the imodulation senvelope :of the input pulses to 1 the :moving iobj'ect selectonislproportional to w, thenithepeakamplitude of theroutput pulselcmodulatiomenvelope :is liepresentedtby the :ordinateflO, If, ihoweven'ithe frequency .iof the;modulationenvelopeof"the .input :pulses to the moving: object selector :is proportionalito to -the peakgamplitudeoftthe output pulse modulation envelope :is :representedby the ordinate .12. H

, .Fig. 20. represents :response :characteristicsat the output of the secondmoving object selector.

This figure shows the same informationas did the graphs'of Figs.-2a"and'2b. i'However,-in this instance,;the response curve varies-as the square ofrw -and hence isagraph obtained bysquaring the .ordinatesof l ig. 2b. ,Attention .is directed to the fact that the ordinates in reach of the graphs of 2 are.not to .thesamescale. 'IIt;is evident by examinationrof Fig. 20 that for equal changes :in radial velocity ;(i. e. 40) 'there is 'a greater. change in thepeak amplitude of the modulationenvelope of pulses forming an output for the secondmovingobject selectorthanthere iswhere only. one moving object selector is used. Consider. a specific gexarnple. Assume that the radio object. locating system is :employed to detectmoving vehicles and that ln;a,d1itiO!lfl'/0 signalsbeingreceived from the moving vehicles, additional signals from other moving objects such as trees-swaying'inithe. wind, etc.,-are alsozre celved. Then the video signals representing the trees, vetc., hereinafter referred. to as ground clutter may have a modulation frequency proportional to whereas .the moving vehicles may be represented ,by video pulses having a modulation envelope proportional-to If these signals are then applied to -j.ust one-moving object selector, said 'moving object selectorwill provide as an output, pulses representing said ground clutter, said pulses having apeak ampli tude proportional .to-ordinate of Fig. 2b. The same moving objectselector will provide output pulsesrepresenting the moving vehicles and lhaving a peak amplitude proportional to ordinate 12 of Fig. 272. As thereis relatively little differ-- ence between .the peak'amplitude of thesignals representing the moving vehiclesancl thosemepresenting the ground clutter, it may be rather difiic'ult to distinguishbetween signals represent ing one and signals representing the other. 7

If however, the pulse vsignals'from the first moving object selectorlare applied to a second moving object selector, then the discrimination is improved appreciably. Forexample, thes ianals representing theground clutter may have apeak amplitude proportional toor'dinate 1 5 of Fig. 2c and the peak amplitude of the signals representing the moving vehicles may be proportional to the ordinate 16 of.Fig. 20. It is quite evident'that it is now much easier to discriminate'between two groups of signals having only slightly difierent modulation frequencies. This becomes especially clear when it is realized-that the ordinates in Fig. 2c are drawn to -a much smallerscale than are the-ordinates of Fig. 2b.

.Reference is'macle to Fig. .3 which iShOWS one embodimentwfzthe presentzinvention employedi n an :ra'dio object :ilocating :system. Video pulsesunanitesting moving hhje'ctsthaving. Ia cyclivarying ;amplitude:.are .applied to input: terminal 20, Videorpulses zare-rapplied simultaneously tto delay-dine 2:! and :tozamplifierZZ. ,Delay lil'lBTZl may consistgof :a delayitype transmission line :of the 501131801381 adesoribed ;in..:a1zcopending, application of ;.GordonD.onald :Forbes and Herbert Shapiro entitled Transmission. Line, 1 Serial No. 608310, filed Augusta, 1945, Patent No.12 23,306.

The :delay, incurred by signals; passing through delayzline 2| substantially equal .to the dnterval between the'video pulses applied thereto. The output .ofxsaid :delay line is then fedtoampliher 23 and the'output therefrom is in turn'applied to amplifier-24. The output issuing from output :amplifier 24 1s then ofopposite polarity from the output .issuing frompthe amplifier 2-2. The gain pfamplifier stages 23 and 2s isadjusted, so that :if non-varying amplitude signals are appliedto input terminal 29, theoutput :from amplifier 25! "will be of equal :amplitude and opposite pOlarity to signals :issuing .from amplifier Z2. Theoutput from amplifier 24 andthetoutput from amplifier -22 arethen simultaneously applied to canceller .25. As'the signals ,applied,

thereto areoiequal andmpposite polarity, canceller .25 .functions to provide --a residual .pulse' substantially proportional to the difierence .in amplitude of .the two .inputs. Thus, .if video signals a plied to input terminal 28 are ,nonvarying ,in amplitude, .the :two inputs to can cellerzfi cancel one another and there ;noout puttherefrom. On.the other hand, if the amplitude of the pulses toinput terminalifi vary from pulse to-pulse,.aresidilalwsignal will be obtained fromcanceller .25 substantially proportional to the difierence .in amplitude .between successive pulses, The .clrcuit which .is described ,may be referredtoasl moving object selectorlfi. .It .is .to be understoodlthat .there are other ways of invertinglone offlthe pulses'to be compared, for example, "the pulses applied -to canc'eller .25 may be of the same polarity and'they maybe inverted therein] by ,anyjwell known-means.

Thesecondmoving objectselectorfl which is identical to moving .object selector 25.1nay .be connected inseriesftherewith. I The output.fr.om canoeller. 2,5,of moving.object-selector 26 is then applied simultaneously toldelay line .28 and amplifier 29gotmoving.objectselector 21. I The 'output [from canceller' 30 .of .movinglobject selector 2'! then. consistsoi video pulses having an .am-

plitude whichis always substantially proportional to'the difference in. amplitude of successive video pulses issuingfrommoving object selector 25, ifrhefoutputirom canceller .30-ofmoving object selector 21.may then beapplied to a suitable indicator-such asacathoderay tube. Because of .the action of the serially connected moving object selectors (as heretofore .fully described the ldiscrimination js greatly improved .and the modulation envelope or the .peak amplitudes of video pulses issuing from moving object selector 2'! varies substantiallyas the second derivative of the modulation envelope of. the video pulses rappliedto input .terminal20.

.Fig. 4 a second embodiment of the present invention which :performs .a function similar 'to theembodiment of Fig. *3 :but which-employs moving objectselectors using a storage "tube :in-

steadbfa delay :line. As heretofore-described,

video pulses may beapplied to a storage tube and an output may be obtained therefrom substantially proportional to the difference in amplitude between successive stored video pulses.

Signals of the same character as those applied to input terminal 29 of Fig. 3 are applied to input terminal 30 of Fig. 4. These signals are in turn applied by means of connection 4| to a storage and cancellation circuit 42. Storage and cancellation circuit 42 comprises a storage tube of the character described and hence provides as an output therefrom video pulses having an amplitude substantially proportional to the difference in amplitude of successive pulses applied to input terminal Ml. These signals are then fed to amplifier :33 and the amplified output therefrom is fed to a second moving object selector comprising a storage and cancellation circuit as which is identical in function to storage and cancellation circuit 42. The output pulses from storage and cancellation circuit is are then fed to and amplified by amplifier 45 and the output therefrom may be applied to an indicator.

As in the instance of the embodiment of Fig. 3, the modulation envelope of the output signals issuing from the second moving object selector is substantially proportional to the second derivative of the modulation envelope of the video pulses applied to input terminal 69.

Although there has been shown a first embodiment using two delay line type moving object selectors and a second embodiment using two storage tube type moving object selectors, it will be apparent to one skilled in the art that there may be combinations thereof. For example, signals may be applied to a delay line type moving object selector and the output therefrom then applied to a storage tube type or vice versa.

Another advantage of using two serially connected moving object selectors is evident when it is realized that the actual signal perceived on an indicator is proportional to the product of the amplitude of the received signal and the ordinate of the response curve. By reference to Fig. 2, it can be seen that at low velocities (in the region of -w=) the ordinate of the response curve shown in Fig. 2c is appreciably smaller than the ordinate of the response curve shown in Fig. 2b. Thus, as signals representing ground clutter have a low modulation frequency (i. e. a: nearly equal to zero), it is apparent that the product of the ordinate of the response curve and the ground clutter signal is much smaller when two moving object selectors are used than when only one is used. Thus, if two serially connected moving object selectors are used, the response to ground clutter which generally has a large amplitude but a low velocity (1 is small) is small and the ground clutter appears as a relatively low amplitude signal on the indicator as compared to signals representing moving objects.

Although there has been here described one embodiment of the present invention, it will be manifest to those skilled in the art that other changes and modifications may be made therein. It is therefore aimed in the appended claims to cover all such modifications as fall within the.

spirit and scope of the invention.

What is claimed is:

1. In a radio object locating system which transmits exploratory pulses of radiant energy and which distinguishes between fixed objects and moving objects by the character of the video pulses obtained from received echo pulses, and wherein fixed objects are evidenced by video pulses having a substantially non-varying amplitude and moving objects are evidenced by pulses having a cyclical variation in amplitude, a first means for selecting pulses from moving objects, said means including a delay type transmission line means for delaying signals of electrical energy for a period of time substantially equal to the interval between exploratory pulses, means for applying a first portion of each video pulse to said delay line, means for algebraically combining the output of said delay line with a second undelayed portion of each of said video pulses to obtain as an output from said first means pulse signals whose amplitude is substantially proportional to the difierence in amplitude between successive video pulses, a second means for selecting pulses from moving objects similar to said first means including a second delay type transmission line, means for applying a first portion of each pulse signal which forms the output from said first means as an input to said second delay line, means for algebraically combining the output of said second delay line with a second undelayed portion of the input to said second means to obtain as an output from said second means pulse signals.

2. In a radio object locating system which transmits exploratory pulses of radiant energy and which distinguishes between fixed objects and moving objects by the character of the video pulses obtained from received echo pulses, and wherein fixed objects are evidenced by video pulses having a substantially non-varying amplitude and moving objects are evidenced by pulses having a cyclical variation in amplitude, a first means responsive to the difierence in amplitude of successive pulses applied thereto for selecting pulses from moving objects, means for applying said video pulses thereto to obtain as an output therefrom, pulse signals whose amplitude is substantially proportional to the difference in amplitude between successive video pulses, a second means responsive to the difference in amplitude of successive pulses applied thereto for selecting pulses from moving objects, and means for applying the output of said first means thereto to obtain pulse signals whose amplitude is substantially proportional to the difference in amplitude betwen successive output pulse signals from said first means, whereby the peak amplitude of the modulation envelope of a plurality of pulse signals from said second means is substantially proportional to the second derivative of the modulation envelope of the plurality of video pulses applied to said first means.

3. In a radio object locating system which transmits exploratory pulses of radiant energy and which distinguishes between fixed objects and moving objects by the character of the video pulses obtained from received echo pulses, and,

wherein fixed objects are evidenced by video pulses having a substantially non-varying amplitude and moving objects are evidenced by pulses having a cyclical variation in amplitude, a first means responsive to the difference in amplitude of successive pulses applied thereto for selectin pulses from moving objects including a' storage tube, means for applying said video pulses to said storage tube for deriving output signal pulses therefrom having an amplitude substantially proportional to the difference in amplitude between successive video pulses, a second means responsive to th difference in amplitude of successive pulses applied thereto for selecting pulses from moving objects including a second storage tube similar to said first storage tube, means for applying the output signals from said first storage tube as an input to said second storage tube and for deriving second output pulse signals therefrom whose amplitude is substantially proportional to the difference in amplitude between successive input pulses to said second means.

4. In a radio object locating system which transmits exploratory pulses of radiant energy and which distinguishes between fixed objects and moving objects by the character of the video pulses obtained from received echo pulses, and wherein fixed objects are evidenced by video pulses having a substantially non-varying amplitude and moving objects are evidenced by pulses having a cyclical variation in amplitude, a plurality of means responsive to the difference in amplitude of successive pulses applied thereto for selecting pulses from moving objects connected in cascade, means for applying said video pulses to the first of said cascade connected selecting means, and. means for deriving output pulse signals from the last of said cascade connected selecting means in response to those video pulses having a cyclical variation in amplitude.

5. In a radio object locating system which transmits periodic exploratory pulses of radiant energy and which distinguishes between fixed objects and moving objects by the character of the echo signals obtained from the received echo pulses, and wherein fixed objects are evidenced by echo signals having a substantially uniform amplitude and moving objects are evidenced by echo signals having a cyclical variation in amplitude, a first means responsive to the difierence in amplitude of successive pulses applied thereto for selecting pulses from moving objects, means for applying said echo signals to said selecting means for deriving output signals therefrom only in response to successive echo signals of difierent amplitudes from a given object, a second means responsive to the difference in amplitude of successive pulses applied thereto for selecting pulses from moving objects, and means for applying the output signals of said first selectin means to said second selecting means for deriving output signals therefrom only in response to successive output signals of diflerent amplitudes from said given object.

6. The apparatus defined in claim 5 wherein said first and second selecting means each includes a cathode ray storage tube.

7. The apparatus defined in claim 5 wherein each of said first and second selecting means includes means for delaying said echo signals for a time equal to the period between successive exploratory pulses, and means for combining each delayed echo signal with a succeedin undelayed echo signal from the same object to obtain a signal in response to the difiference in amplitudes between said delayed and undelayed signals.

FREDERIC CUNNINGHAM, J R.

References Cited in the file of this patent UNITED STATES PATENTS 

